Circular fluorescent lamp

ABSTRACT

This invention provides a compact, efficient and well-designed circular fluorescent lamp with high output power. The circular fluorescent lamp has less non-luminant region and good characteristics of luminous intensity distribution, and keeps high level of lamp lumen output. Circular arc tubes are disposed concentrically on the same plane. Electrodes are attached to the tube-end parts of the one side of the circular arc tubes respectively, and the other tube-end parts of non-electrode side are sealed. The circular arc tubes are joined each other near the tube-end parts of the non-electrode side with a bridge-jointed portion, so that a discharge path is formed between the electrodes inside the arc tubes. The tube outer diameter of the circular arc tubes is 14 mm, the circle outer diameter of the outer circular arc tube is 150 mm, and the circle inner diameter of the inner arc tube is 90 mm. The distance measured along the center line of the tube from the point corresponding radially to the end part of the bridge-jointed portion to the sealed tube-end part of the outer circular arc tube is 11 mm, and the distance measured along the center line of the tube from the point corresponding radially to the end part of the bridge-jointed portion to the sealed tube-end part of the inner circular arc tube is 6 mm.

FIELD OF THE INVENTION

This invention relates to a circular fluorescent lamp wherein pluralcircular arc tubes are concentrically disposed.

BACKGROUND OF THE INVENTION

Circular fluorescent lamps have been commonly used, mainly forresidential lightings. In order to obtain higher output power, at leasttwo circular fluoresent lamps are disposed unevenly for a specializedlighting equipment. Such lighting equipments would become thicker andbigger, so it is not economical. Another problem is that the designingof the appliances will be limited. In another case, a circularfluorescent lamp is used and the output of the lamp is as high as thesum of the above-mentioned plural circular fluorescent lamps. However,the circular fluorescent lamp and the equipment become bigger, andeconomical and design problems will also occur.

In order to solve these problems of the conventional circularfluorescent lamps, Published Unexamined (Kokai) Japanese PatentApplication No. Hei 2-61956, and No. Hei 6-203798 disclose circularfluorescent lamps respectively. Such circular fluorescent lamps aremanufactured by disposing two compact glass circular arc tubes 21 and 22concentrically on the same plane, joining the tubes with a glass tubebridge-jointed portion 23 and forming a discharging path inside thetubes (see FIG. 25). In the FIG. 25, numbers 24 and 25 indicateelectrodes. Several examinations were carried out with these fluorescentlamps. It was found that in these circular fluorescent lamps, the areaof a non-luminous portion including an electrode mounting part aroundthe circumference, namely a distance K in FIG. 25, would be increasedcompared to that of the conventional circular fluorescent lamps. Even ifa lamp base is formed, the characteristics of luminous intensitydistribution around the circumference of the lamp will be deteriorated.And a larger base will cause problems in designing. The lamp has acompact configuration and is lighted up with a relatively heavy load.Thus the coldest temperature inside the arc tube, which determines themercury vapor pressure, rises over the optimum range of 40°-50° C. As aresult, the lamp lumen output becomes lower than the maximum value as afluorescent lamp. In addition, the lamp lumen output will bedeteriorated when the area of the non-luminous portion is increased.

SUMMARY OF THE INVENTION

In order to solve the above-mentioned problems of conventionaltechniques, this invention provides a compact, efficient andwell-designed circular fluorescent lamp with high output power. Thecircular fluorescent lamp of this invention also has less non-luminousarea and good characteristics of luminous intensity distribution, andkeeps a high level of lamp lumen output around the lamp circumference.

In order to achieve the purposes, a first circular fluorescent lamp ofthis invention has plural circular arc tubes disposed concentrically,and each of the tubes has an electrode at one tube-end part and a sealedportion at the other tube-end part. The tubes are joined near the sealedportion with a bridge-jointed portion, and thus a discharge path isformed inside the tubes. When the distance L₁ (mm) is measured along thecenter line of the tube from the point corresponding radially to the endpart of the bridge-jointed portion to the sealed tube-end part of theouter circular arc tube, and the distance L₂ (mm) is measured along thecenter line of the tube from the point corresponding radially to the endpart of the bridge-jointed portion to the sealed tube-end part of theinner circular arc tube, L₁ is longer than L₂.

It is preferable in this circular fluorescent lamp that the pluralcircular arc tubes are disposed on the same plane.

It is also preferable that L₁ ≧1.3L₂ holds true.

It is also preferable that the tube-end part of the electrode side ofthe outer circular arc tube is longer than that of the inner circulararc tube.

It is also preferable that the configuration of the tips of thebridge-jointed portions are essentially symmetric to the center axis ofthe circular arc tubes.

It is also preferable that holding parts for a bending process areprovided at the outer surfaces of the both tube-end parts of thecircular arc tubes. Preferably in this case, grooves are formed on atleast the inner surfaces of the tips of the non-electrode side.

It is also preferable that a lamp base is provided to surround thetube-end parts of the electrode side of the circular arc tubes.

It is also preferable that a lamp base is provided to surround the bothtube-end parts of the circular arc tubes while the tube-end part of thenon-electrode side of the outer arc tube is exposed to the open air.Preferably in this case, a thermal shielding part is provided with thelamp base in order to shield the heat between the tube-end parts of theelectrode side and the tube-end parts of the non-electrode side.

It is also preferable that the tube-end parts of the non-electrode sideof the circular arc tubes are sealed with stems.

According to a second embodiment, a circular fluorescent lamp of thisinvention has plural circular arc tubes disposed concentrically, andeach of the tubes has an electrode at one tube-end part, and a sealedportion at the other tube-end part. The tubes are joined near the sealedportions with a bridge-jointed portion to form one discharge path insidethe tubes, and a coldest spot is formed at the other tube-end part ofthe non-electrode side. When the distance L(mm) is measured along thecenter line of the tube from the point corresponding radially to the endpart of the bridge-jointed portion to the sealed tube-end part, and thetube outer diameter of the arc tube is d(mm), 0.5d≦L≦1.3d holds true.

It is preferable in this second embodiment that the plural arc tubes aredisposed on the same plane.

It is also preferable that holding parts for a bending process areprovided at the outer surfaces of the both tube-end parts of thecircular luminous tubes. Preferably in this case, grooves are formed atleast at the inner surfaces of the tips of the non-electrode side.

It is also preferable that the non-electrode tube-end parts of thecircular arc tubes are sealed with stems.

According to a third embodiment, a circular fluorescent lamp of thisinvention has plural arc tubes disposed concentrically, and each of thetubes has an electrode at one tube-end part and a sealed portion at theother tube-end part. The tubes are joined near the sealed portion with abridge-jointed portion to form one discharge path inside the tubes, anda coldest spot is formed at the tube-end part of the non-electrode side.And the circular fluorescent lamp is provided with a lamp base whichsurrounds at least one side of the both tube-end parts, and the coldestspot is exposed to the open air.

It is preferable in the third embodiment that the plural circular arctubes are disposed on the same plane.

It is also preferable that when the distance L(mm) is measured along thecenter line of the tube from the point corresponding radially to the endpart of the bridge-jointed portion to the sealed tube-end part and thetube outer diameter of the circular arc tube is d(mm), 0.5d≦L≦1.3d holdstrue.

It is also preferable that the lamp base is separated from the coldestspot of the non-electrode side.

It is also preferable that the lamp base surrounds the tube-end parts ofthe electrode side and the coldest spot of the other tube-end part, andthat a vent is provided to the lamp base so that the coldest spot isexposed to the open air.

It is also preferable that the coldest spot is formed at the outercircular arc tube.

It is also preferable that holding parts for a bending process areprovided at the outer surfaces of the both end parts of the circular arctubes. Preferably in this case, grooves are formed on the inner surfacesof the tips of the non-electrode side.

It is also preferable that the end parts of the non-electrode side aresealed with stems.

According to a fourth embodiment, a circular fluorescent lamp of thisinvention has plural circular arc tubes disposed concentrically, andeach of the tube has an electrode at one tube-end part and a sealedportion of the other tube-end part. The tubes are joined near thetube-end parts of the non-electrode side with a bridge-jointed portionto form a discharge path inside the tubes, while a coldest spot isformed at the tube-end part of the non-electrode part and a lamp base isprovided to surround at least one side of the both tube-end parts. And athermal shielding part is provided to the lamp base in order to shieldthe heat between the tube-end parts of the electrode side and those ofthe non-electrode side.

It is preferable in the fourth embodiment that the plural circular arctubes are disposed on the same plane.

It is also preferable that when the distance L(mm) is measured along thecenter line of the tube from the point corresponding radially to the endpart of the bridge-jointed portion to the sealed tube-end part and thetube outer diameter of the circular arc tube is d(mm), 0.5d≦L≦1.3d holdstrue.

It is also preferable that a lamp base is provided to surround thetube-end parts of the electrode side and the coldest spot of the othertube-end part, and a vent is formed on the lamp base so that the coldestspot is exposed to the open air.

It is also preferable that holding parts for a bending process areprovided at the outer surfaces of the both end parts of the circular arctubes. Preferably in this case, grooves are formed at least at the innersurfaces of the tips of the non-electrode side.

It is also preferable that the tube-end parts of the non-electrode sideof the circular arc tubes are sealed with stems.

According to the first embodiment, the circular fluorescent lamp hasplural arc tubes disposed concentrically, and each of the tubes has anelectrode at one end part and a sealed portion of the other tube-endpart. The tubes are joined near the tube-end parts of the non-electrodeside to form a discharge path inside the tubes with the bridge-jointedportion. When the distance L₁ (mm) is measured along the center line ofthe tube from the point corresponding radially to the end part of thebridge-jointed portion to the sealed tube-end part of the outer circulararc tube and the distance L₂ (mm) is measured along the center line ofthe tube from the point corresponding radially to the end part of thebridge-jointed portion to the sealed tube-end part of the inner circulararc tube, L₁ is longer than L₂. Therefore, a coldest spot is formed onthe end part of the non-electrode side of the outer circular arc tube.In addition, the temperature can be easily controlled to provide thebest mercury vapor pressure which is corresponding to the maximum valueof the lamp lumen output.

And according to the preferable example of the first embodiment in whichthe plural circular arc tubes are disposed on the same plane, thelighting equipment can be thinner.

According to the preferable example of the first embodiment in which L₁≧1.3L₂ holds true, a coldest spot to provide the best mercury vaporpressure can be accurately formed at the tip of the bridge-jointedportion side of the outer circular arc tube.

According to the preferable example of the first embodiment in which thetube-end part of the electrode side of the outer circular arc tube islonger than that of the inner circular arc tube, the effective luminouslength becomes longer and as a result, lamp lumen output can beincreased. In addition, if the effective luminous length of the circulararc tube is longer, the non-luminous area of the circular fluorescentlamp becomes smaller. Therefore, the characteristics of the luminousintensity distribution can be improved and a compact and well-designedcircular fluorescent lamp can be provided.

According to the preferable example of the first embodiment in which theconfigurations of the tips of the bridge-jointed portion side areessentially symmetric to the center axis of the circular arc tube, thestrength of the tip of the bridge-jointed portion side will not belowered.

According to the preferable example of the first embodiment in whichholding parts for a bending process are provided at the outer surfacesof the both tube-end parts of the circular arc tubes, the holding partsfor a bending process of the circular arc tubes can be held securelywhile manufacturing a lamp. Therefore, the bending accuracy of thecircular arc tubes can be improved. In addition, according to thepreferable example in which grooves are formed at least at the innersurfaces of the tips of the non-electrode side, the coldest spots areformed in the grooves to keep the temperature at the best level.

According to the preferable example of the first embodiment in which alamp base is provided to surround the tube-end parts of the electrodeside of the circular arc tubes, the heat of the electrodes is nottransferred to the other tube-end parts of the non-electrode side, sothe temperature of the coldest spot does not rise excessively from thebest temperature region, and therefore, the lamp lumen output is notlowered.

According to the preferable example of the first embodiment in which alamp base is provided to surround the both tube-end parts of thecircular arc tubes and those of the non-electrode side, and the tube-endpart of the outer circular arc is exposed to the open air, the pluralcircular arc tubes can be securely held. In addition, the temperature ofthe coldest spot does not excessively rise. As a result, the lamp lumenoutput will not be lowered. And according to the preferable example inwhich a thermal shielding part is provided in order to shield the heatbetween the tube-end parts of the electrode side and those of thenon-electrode side, transference of heat from the electrodes to thetube-end parts of the other side where the coldest spot is formed can becontrolled. As a result, it is possible to prevent the excessive rise ofthe temperature of the coldest spot more securely, so the lamp lumenoutput will not be lowered.

According to the preferable example of the first embodiment of thisinvention, in which the tube-end parts of the non-electrode side of thecircular arc tubes are sealed with stems, the strength of the tube-endparts of the non-electrode sides is improved compared to that of theconventional tubes. Conventionally, tube-end parts are formed bypartially melting arc tubes. If the diameter of the arc tube is large,the tube-end parts of the non-electrode side become thin. On the otherhand, the tube-end parts can be securely sealed by stems and also,inconvenience like cracking of the tube-end parts will be preventedduring the tube bending process and after the lamp is completed. Inaddition, the process of manufacturing by sealing with stems is simplecompared to that of the prior art, and thus the arc tubes of thisinvention can be manufactured using the conventional facilities.

The circular fluorescent lamp of the second embodiment of this inventionhas plural circular arc tubes disposed concentrically, and each of thetubes has an electrode at one tube-end part and a sealed portion of theother tube-end. And the tubes are joined near the other tube-end partswith the bridge-jointed portion to form a discharge path inside thetubes while coldest spots are formed at the tube-end parts of thenon-electrode parts. When the distance L(mm) is measured along thecenter line of the tube from the point corresponding radially to the endpart of the bridge-jointed portion to the sealed tube-end part and theouter diameter of the circular arc tube is d(mm), 0.5d≦L≦1.3d holdstrue. Therefore, the temperature of the coldest spots can be kept in theregion where the mercury vapor pressure in the arc tube is the best, andthe maximum value of the lamp lumen output can be easily obtained.

The circular fluorescent lamp of the third embodiment of this inventionhas plural circular arc tubes disposed concentrically, and each of thetubes has an electrode at one tube-end part and a sealed portion at theother tube-end part. And the tubes are joined near the tube-end parts ofthe non-electrode side with the bridge-jointed portion to form a pathinside the tubes, while coldest spots are formed at the other tube-endparts. In addition, a lamp base is provided to surround the tube-endparts of the electrode side and/or those of the non-electrode side, andthe coldest spots are exposed to the open air. The structure preventsthe heat transference from the electrodes to the side of thebridge-jointed portion, and therefore, the temperature of the coldestspot does not rise excessively from the best temperature region. As aresult, the lamp lumen output will not be lowered.

According to the preferable example of the third embodiment in which thelamp base is provided separated from the coldest spots in thenon-electrode side, the coldest spots can be exposed to the open air.

According to the preferable example of the third embodiment in which thelamp base is provided to surround the tube-end parts of the electrodeside and the coldest spots of the other side, and a vent is formed onthe lamp base so that the coldest spots are exposed to the open air, thecoldest spots can be exposed to the open air while the plural circulararc tubes are held securely.

According to the preferable example of the third embodiment in which acoldest spot is formed on the outer circular arc tube, the non-luminousarea of the tube-end parts of the tubes becomes smaller, and a higherlumen output value can be obtained.

The circular fluorescent lamp of the fourth embodiment of this inventionhas plural circular arc tubes disposed concentrically, and each of thetubes has an electrode at one tube-end part and a sealed portion of theother tube-end part. And the tubes are joined near the tube-end parts ofthe non-electrode side with the bridge-jointed portion to form adischarge path inside the tubes, while coldest spots are formed at theother tube-end parts. In addition, a lamp base is provided to surroundthe tube-end parts of the electrode side and/or those of thenon-electrode side, and thermal shielding parts are provided for thelamp base to reduce the heat transference between the tube-end parts ofthe electrode side and those of the non-electrode side, so that heattransference from the electrodes to the coldest spot can be controlled.As a result, it is possible to prevent the temperature of the coldestspots from rising excessively from the best temperature region, and thelamp lumen output will not be lowered.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially cutaway view in elevation of the circularfluorescent lamp of Example 1 of this invention.

FIG. 2 is a partially cutaway view in elevation to show the structurearound the bridge-jointed portion of the circular fluorescent lamp ofFIG. 1.

FIG. 3 is an elevational view to show the structure around a circularfluorescent lamp manufactured to be compared with that of Example 1.

FIG. 4 is an elevational view to show another structure around thebridge-jointed portion of the circular fluorescent lamp of Example 1 ofthis invention.

FIG. 5 is an elevational view to show the structure around thebridge-jointed portion of a circular fluorescent lamp manufactured inExample 1 to be compared with that shown in FIG. 4.

FIG. 6 is an elevational view to show a structure around thebridge-jointed portion of the circular fluorescent lamp of Example 3 ofthis invention.

FIG. 7 is an elevational view to show another structure around thebridge-jointed portion of the circular fluorescent lamp of Example 3 ofthis invention.

FIG. 8 is an elevational view to show the structure around thebridge-jointed portion of a circular fluorescent lamp manufactured as acomparative example to the lamps shown in FIGS. 6 and 7 in Example 3 ofthis invention.

FIG. 9 is a partially cutaway view in elevation to show the structurearound the bridge-jointed portion of the circular fluorescent lamp ofExample 4 of this invention.

FIG. 10 is a partially cutaway view in elevation to show a structure ofthe circular fluorescent lamp of Example 5 of this invention.

FIG. 11 is a partially cutaway view in elevation to show anotherstructure of the circular fluorescent lamp of Example 5 of thisinvention.

FIG. 12 is a partially cutaway view in elevation of a circularfluorescent lamp of Example 6 of this invention.

FIG. 13 is a partially cutaway view in elevation to show the structurearound the bridge-jointed portion of the circular fluorescent lamp shownin FIG. 12.

FIG. 14 is an elevational view to show the structure around a circularfluorescent lamp manufactured to be compared with the ones of Examples 7and 8.

FIG. 15 is an elevational view to show a structure around thebridge-jointed portion of the circular fluorescent lamp of Example 7 ofthis invention.

FIG. 16 is an elevational view to show another structure around thebridge-jointed portion of the circular fluorescent lamp of Example 7 ofthis invention.

FIG. 17 is a partially cutaway view in elevation to show a structurearound the bridge-jointed portion of the circular fluorescent lamp ofExample 8 of this invention.

FIG. 18 is an elevational view to show a structure around thebridge-jointed portion of the circular fluorescent lamp of Example 9 ofthis invention.

FIG. 19 is an elevational view to show another structure around thebridge-jointed portion of the circular fluorescent lamp of Example 9 ofthis invention.

FIG. 20 is a partially cutaway view in elevation to show a structure ofthe circular fluorescent lamp of Example 10 of this invention.

FIG. 21 is a partially cutaway view in elevation to show anotherstructure of the circular fluorescent lamp of Example 10 of thisinvention.

FIG. 22 is a partially cutaway view in elevation to show a structure ofthe circular fluorescent lamp of Example 11 of this invention.

FIG. 23 is a partially cutaway view in elevation to show a structure ofthe circular fluorescent lamp of Example 12 of this invention.

FIG. 24 is a partially cutaway view in elevation to show anotherstructure of the circular fluorescent lamp of Example 12 of thisinvention.

FIG. 25 is a partially cutaway view in elevation of a conventionalcircular fluorescent lamp.

DETAILED DESCRIPTION OF THE INVENTION

This invention will be explained in detail with reference to theattached figures and the following examples.

EXAMPLE 1

FIG. 1 is a partially cutaway view in elevation of the circularfluorescent lamp of Example 1 of this invention, and FIG. 2 is apartially cutaway view in elevation to show the structure around thebridge-jointed portion of the circular fluorescent lamp of FIG. 1. Asshown in these figures, the two glass circular arc tubes (1, 2) aredisposed concentrically on the same plane, and at the tube-end parts ofthe circular arc tubes (1, 2) electrodes (3, 4) are respectivelyattached. Each of the other tube-end parts (11, 12) of the tubes (1, 2)is sealed. The parts adjacent to the tube-end parts (11, 12) are joinedwith a glass tube, namely, bridge-jointed portion 5, so that a dischargepath is formed between the electrodes (3, 4) inside the luminant tubes.The inner surfaces of the circular arc tubes (1, 2) are coated with rareearth fluorophor 6, and mercury and rare gas (200-500 Pa) for startingand buffering, e.g., argon and neon, are sealed in the tubes. Themercury can be an amalgam alloy like zinc-mercury.

The circular fluorescent lamp of this invention has a compact shape, inwhich the tube outer diameter of the circular arc tubes (1, 2) is 14 mm,the circle outer diameter of the outer circular arc tube 1 is 150 mm,and the circle inner diameter of the inner arc tube 2 is 90 mm. The lampis designed to light on at 25 W of lamp power input.

L₁ is the distance measured along the center line of the tube from thepoint corresponding radially to the end part of the bridge-jointedportion 5 to the sealed tube-end part 11 of the outer circular arc tube1, and L₂ is the distance measured along the center line of the tubefrom the point corresponding radially to the end part of thebridge-jointed portion 5 to the sealed tube-end part 12 of the innercircular arc tube 2. L₁ is 11 mm and L₂ is 6 mm. The distance betweenthe centers of the both ends of the outer circular arc tube 1, which isrepresented as 1, is 18 mm. When the circular fluorescent lamp waslighted up at 25 W using an inverter circuit of 50 kHz, a high lumenoutput value, namely 1620 lm, with a luminous color of 3000 kelvin ofcolor temperature, was obtained. Temperatures of the circular arc tubes(1, 2) were measured and it was found that a coldest spot was formed onthe tube-end part 11 of the outer circular arc tube 1, and thetemperature of the coldest spot was 45° C. This corresponds to the bestmercury vapor pressure in which almost the maximum lumen output valuecan be obtained when 25 W is lighted up (when the room temperature is25° C.).

FIG. 25 shows a conventional circular fluorescent lamp that wasmanufactured to measure characteristics of a lamp. L₁ ' is the distancemeasured along the center line of the tube from the point correspondingradially to the end part of the bridge-jointed portion 23 to the sealedtube-end part 26 of the outer circular arc tube 21, and L₂ ' is thedistance measured along the center line of the tube from the pointcorresponding radially to the end part of the bridge-jointed portion 23to the sealed tube-end part 27 of the inner circular arc tube 22. Thevalue of L₁ ' is equal to that of L₂ '. According to this lamp, thedistance for L₁ ' and L₂ ', that of the temperature of the coldest spotsof the tips in the best region was 13 mm. This lamp was lighted up at 25W using an inverter circuit of 50 kHz as mentioned above, and the lamplumen output was 1490 lm. In other words, the circular fluorescent lampof this invention (FIGS. 1 and 2) had a higher lumen output value thanthat of the conventional circular fluorescent lamp shown in FIG. 25, andthe difference was 130 lm (about 9%). Such a high value was obtainedsince the distance L₁ was longer than the distance L₂. Thus a coldestspot was formed at the tube-end part 11 of the non-electrode side of theouter circular arc tube 1, and the temperature was easily controlled tobe the best mercury vapor pressure which corresponded to the maximumvalue of the lamp lumen output. And compared to the prior art of FIG.25, the circular fluorescent lamp of this invention had less area whichdid not luminesce. Therefore, the characteristic of the luminousintensity distribution along the circular circumference was improved,and the improvement also contributed to the design of the lamp.

As shown in FIGS. 1 and 2, the electrode tube-end part of the outercircular arc tube 1 is displaced to be longer than that of the innercircular arc tube 2. In other words, the difference between the lengthof the tubes is represented as S. For reference, another lamp wasmanufactured to measure the characteristics of the lamps. FIG. 3 shows alamp in which an electrode 3 of the circular arc tube 1 and anotherelectrode 4 of the circular arc tube 2 were disposed in parallel. Whenthe lamp was lighted up at 25 W of lamp input power using an invertercircuit of 50 kHz, the lamp lumen output was 1580 lm. In other words,the circular fluorescent lamp of this invention had longer effectiveluminous light and the lumen output value became higher, and thedifference was 40 lm (3%). In this case, too, the non-luminescing areabecame smaller and the characteristic of luminous intensity is improved,and the improvement clearly contributes to the design of the lamp.

The tube-end parts (11, 12) of the bridge-jointed portion 5 of thecircular arc tubes (1, 2) can be projected as shown in FIG. 4. The shapeof the tube-end parts (11, 12) of the tube (1, 2) should not beasymmetric to the center axis of the tube as shown in FIG. 5, since thestrength of the glass tip is significantly lowered. In any event, a tipof the tube should be processed to be symmetric to the center axis ofthe tube.

EXAMPLE 2

In this example, a compact circular fluorescent lamp with 60 W of lampinput was manufactured. The structure of the lamp was basically the sameas that of Example 1 (FIGS. 1 and 2). The tube outer diameter of thecircular arc tubes (1, 2) was 20 mm, the circle outer diameter of theouter circular arc tube 1 was 240 mm, the circle inner diameter of theinner circular arc tube 2 was 155 mm, the distance L₁ measured along thecenter line of the tube from the point corresponding radially to the endpart of the bridge-jointed portion 5 to the sealed tube-end part 11 ofthe outer circular arc tube 1 was 17 mm, the distance L₂ measured alongthe center line of the tube from the point corresponding radially to theend part of the bridge-jointed portion 5 to the sealed tube-end part 12of the inner circular arc tube 2 was 10 mm, and the center distance 1 ofthe both ends of the circular arc tube 1 was 22 mm. When the lamp waslighted up at 60 W of lamp input using an inverter circuit of 50 kHz, ahigh lumen output value (4530 lm) was obtained.

Related to the lamps of Examples 1 and 2, the lumen output values werefurther measured, especially varying the distance L₁. When the maximumlumen output value was obtained, 0.5d≦L₁ ≦3d holds, where d was the tubeouter diameter of the circular arc tubes (1, 2). And it was found outthat the relation between L₁ and L₂ should be L₁ ≧1.3L₂ so that acoldest spot to provide the best mercury vapor pressure is securelyformed on the tube-end 11 of the outer circular arc tube 1.

EXAMPLE 3

This example refers to a circular fluorescent lamp which has the samestructure of the lamps shown in Examples 1 and 2, except that a lampbase is attached.

FIG. 6 is an elevational view to show a structure around thebridge-jointed portion of the circular fluorescent lamp of Example 3 ofthis invention, and FIG. 7 is an elevational view to show anotherstructure around the bridge-jointed portion of the circular fluorescentlamp of Example 3 of this invention. FIG. 6 shows a circular fluorescentlamp in which the tube-end parts of the circular arc tubes 1 and 2(electrodes 3, 4 side) are surrounded with a lamp base 7. The tube-endparts 11 and 12 are separated from the lamp base 7 and exposed to theopen air. On the other hand, FIG. 7 shows a circular fluorescent lamp inwhich tube-end parts in the electrodes (3, 4) side and the othertube-ends (11, 12) are surrounded with a lamp base 8. In this case, avent 10 is provided for the lamp base 8 so that the tube-end part (thecoldest spot) of the outer circular arc tube 1 is exposed to the openair. As mentioned above, the lamp bases 7 and 8 are attached while thetube-end part of the non-electrode side (the coldest spot) 11 of theouter circular arc tube 1 is exposed to the open air. The heat of theelectrodes 3 and 4 will not be transferred to the tube-end part 11, sothe temperature of the coldest spot does not excessively rise.Therefore, the lamp lumen output will not be lowered. Especially, whenthe lamp base 8 is attached to surround both of the tube-end parts ofthe electrodes (3, 4) side and the other tube-end parts (11, 12), thecircular arc tubes 1 and 2 can be stable and well-kept.

In FIG. 8, a lamp base 9 is not formed with a vent, thus the tube-endpart (the coldest spot) 11 of the outer circular arc tube 1 is notexposed to the open air. When this lamp base 9 is used to surround theboth tube-end parts of the circular arc tubes 1 and 2, L₁ should belonger than those of Examples 1 and 2 in order to prevent the excessivetemperature increase of the coldest spot at the tips of the tube-endpart when a lamp is completed. As a result, the effective luminouslength of a lamp with the lamp base 9 in FIG. 8 becomes shorter thanthose of lamp bases 7 and 8 in FIGS. 6 and 7, and the lamp lumen outputis lowered.

EXAMPLE 4

FIG. 9 is a partially cutaway view in elevation to show a structurearound the bridge-jointed portion of the circular fluorescent lamp ofExample 4 of this invention. As shown in FIG. 9, a lamp base 13 isattached surrounding the tube-end parts of the electrode (3, 4) side andthe other tube-end parts (11, 12) of the circular arc tubes (1, 2). Thelamp base 13 is formed with a vent 10 and a thermal shielding plate 16.The lamp base 13 allows the tube-end part (the coldest spot) 11 of theouter circular arc tube 1 to be exposed to the open air, and the thermalshielding plate 16 shields the heat between the tube-end parts of theelectrodes (3, 4) side and the other tube-end parts (11, 12). Thethermal shielding plate 16 controls the transference of heat from theelctrodes (3, 4) to the other tube-end part 11 of the circular arc tube1 where the coldest spots are formed. Thus it is possible to prevent thetemperature of the coldest spot from rising excessively above the besttemperature region more securely compared to the case of Example 3. As aresult, the lamp lumen output will not be lowered.

EXAMPLE 5

FIG. 10 is a partially cutaway view in elevation to show a structurearound the bridge-jointed portion of the circular fluorescent lamp ofExample 5 of this invention. As shown in FIG. 10, holding parts 20 for abending process are provided at the outer surfaces of the both tube-endparts of the circular arc tubes 1 and 2. And grooves 19 are formed onthe inner surfaces of the tips of at least the tube-end parts (11, 12)of the non-electrode side. Other features are the same as those ofExample 4 (FIG. 9), so the explanation is omitted. According to thisstructure, the holding parts 20 for the bending process can be firmlyheld during the lamp manufacturing process, and thus the accuracy forthe bending process of the circular arc tubes (1, 2) will be improved.In addition, coldest spots will be formed at the grooves 19, and thetemperature can be kept at the optimum level.

As shown in FIG. 11, the tube-end parts (11, 12) of the non-electrodeside can be sealed with a stem in the same way as the tube-end parts ofthe electrode side. Thus the strength of the tube-end parts (11, 12)will be improved compared to the case shown in FIG. 10. In FIG. 10, thetube-end parts are formed by partially melting the arc tubes. Therefore,tube-end parts (11, 12) of arc tubes with a larger tube diameter becomethinner. On the other hand, if the tube-end parts are sealed with stems,some problems like cracks of the tube-end parts can be prevented duringthe bending process and after the lamp is completed. In addition, themanufacturing method using stems to seal is relatively simple comparedto the case shown in FIG. 10, and thus the tubes can be manufacturedusing conventional facilities.

EXAMPLE 6

FIG. 12 is a partially cutaway view in elevation to show the circularfluorescent lamp of Example 6 of this invention. FIG. 13 is a partiallycutaway view in elevation to show the structure around thebridge-jointed portion of the circular fluorescent lamp shown in FIG.12. FIGS. 12 and 13 show that two glass circular arc tubes (1, 2) aredisposed concentrically on the same plane, and electrodes (3, 4) areattached to the tube-end parts of one side of the circular arc tubes (1,2) respectively. The other tube-end parts (11, 12) of the circular arctubes (1, 2) are sealed. The circular arc tubes 1 and 2 are joined nearthe tube-end parts (11, 12) with a glass bridge-jointed portion 5 inorder to form a discharge path between the electrodes (3, 4) inside thearc tubes. Rare earth fluorophor 6 is coated on the inner surfaces ofthe arc tubes (1, 2). Mercury and rare gas (200-500 Pa) as astarting-gas as well as a buffer gas (e.g., argon and neon) are sealedin the tubes. The mercury can be an amalgam like zinc-mercury.

The circular fluorescent lamp of this example has a compact shape, inwhich the tube outer diameter of the circular arc tubes (1, 2) is 14 mm,the circle outer diameter of the outer circular arc tube 1 is 150 mm,and the circle inner diameter of the inner arc tube 2 is 90 mm. The lampis designed to light on at 25 W of lamp power input.

L is the distance measured along the center line of each of the tubes(1, 2) from the point corresponding radially to the end part of thebridge-jointed portion 5 to each of the sealed tube-end parts (11, 12).The center distance between the tube-end parts of the inner circular arctube 2 is represented as l'. L is 13 mm and l' is 18 mm. When thecircular fluorescent lamp was lighted up at 25 W using an invertercircuit of 50 kHz, a high lumen output value, namely 1490 lm, withluminous color of 3000 kelvin of color temperature was obtained.Temperatures of the circular arc tubes (1, 2) were measured and thecoldest spots were found to be formed on the tube-end parts (11, 12).The temperatures of the coldest spots were 45° C. This corresponds tothe best mercury vapor pressure in which almost the maximum lumen outputvalue can be obtained when 25 W is lighted up (when the room temperatureis 25° C.). After various examinations, it was found that the distance Lshould be limited to 0.5d≦L≦1.3d where the tube outer diameter isrepresented as d. As a result, the temperature of the coldest spots waskept in a region where the mercury vapor pressure inside the arc tubesis best, and the maximum value of the lamp lumen output was easilyobtained.

EXAMPLE 7

This example refers to a circular fluorescent lamp which is identical tothe lamp in Example 6 except that a lamp base is attached.

A circular fluorescent lamp shown in FIG. 14 was manufactured in whichthe whole tube-end parts of the circular arc tubes (1, 2) are surroundedby a conventional lamp base 28, and the characteristics were measured.And it was found that the lumen output value of this lamp was 1260 lm,while that value of a naked lamp without a lamp base is 1490 lm. Inother words, the lamp lumen output was excessively lowered, because thetemperature of the coldest spots, and thus the mercury vapor pressure,rose excessively from the best region.

Based on the result, two types of circular fluorescent lamps weremanufactured. A lamp in FIG. 15 has a lamp base 29 which is surroundingthe tube-end parts of the electrodes (3, 4) side of the circular arctubes (1, 2), while the other tube-end parts (the coldest spots) 11 and12 are separated from the lamp base 29 and exposed to the open air.Another lamp in FIG. 16 has a lamp base 30 which is surrounding thetube-end parts of the electrodes (3, 4) side and the tube-end parts (11,12) of the other side of the circular arc tube (1, 2), and the tube-endparts (11, 12) are exposed to the open air through a vent 10 formed onthe lamp base 30. The lamp lumen output value of the lamp shown in FIG.15 was 1575 lm, and the lamp lumen output value of the lamp shown inFIG. 16 was 1520 lm. Both values were approximate to that of a nakedlamp without a lamp base. In other words, the lamp lumen output was notlowered so much, since the heat of the electrodes (3, 4) was nottransferred to the tube-end parts (11, 12), so the temperature of thecoldest spots did not rise excessively from the best temperature region.

EXAMPLE 8

This example refers to a circular fluorescent lamp with the ratedelectric power of 60 W.

The structure of the circular fluorescent lamp of this example isbasically as same as that of Example 6 (see FIGS. 12 and 13). And thecircular fluorescent lamp of this example has a lamp base 29 which issurrounding the tube-end parts of the electrodes (3, 4) side of thecircular arc tubes (1, 2), while the other tube-end parts (the coldestspots) 11, 12 are separated from the lamp base 29 to be exposed to theopen air (see FIG. 15).

The tube outer diameter of the circular arc tubes (1, 2) is 20 mm, thecircle outer diameter of the outer circular arc tube 1 is 240 mm, circleinner diameter of the inner circular arc tube 2 is 155 mm, the distanceL measured along the center line of each of the tubes (1, 2) from thepoint corresponding radially to the end part of the bridge-jointedportion 5 to each of the sealed tube-end parts (11, 12) is 19 mm, andthe center distance l of the both tube-end parts of the circular arctube 2 is 22 mm. When this lamp was lighted up with a lamp input of 60 Wusing an inverter circuit of 50 kHz, a high lamp lumen output, namely4390 lm, was obtained.

A conventional lamp base 28 shown in FIG. 14 was used for the circularfluorescent lamp of this example, and further consideration was given tothe structure of the lamp base 28 surrounding the whole tube-end partsof the circular arc tubes 1 and 2. Even if the whole tube-end parts ofthe tubes are surrounded by the lamp base, it is possible to prevent thetemperature of the coldest spots from rising excessively from the bestregion. For this purpose, a lamp base which is formed with a thermalshielding plate 16 made of a thermal shielding part to shield theelectrode side from the other tube-end parts is used. (see FIG. 17).Then, the transference of heat from the tube-end parts of the electrode(3, 4) side to the coldest spots (the other tube-end parts 11, 12) iscontrolled, and the lamp lumen output is not lowered. Specifically, thelamp lumen output value was 1260 lm when the lamp base 28 of FIG. 13 wasused, while the value was 1420 lm when the lamp base 31 with the thermalshielding plane 16 was used. A similar thermal shielding effect wasobtained when the lamp base 30 with the vent 16 of FIG. 16 was used.Specifically, the lamp lumen output value was 1520 lm as mentioned abovewhen the lamp base 30 with only the vent 10 was used. But the value was1560 lm (40 lm higher) when a thermal shielding plate was furtherprovided.

EXAMPLE 9

FIG. 18 is an elevational view to show a structure around thebridge-jointed portion of the circular fluorescent lamp of Example 9 ofthis invention. FIG. 19 is an elevational view to show another structurearound the bridge-jointed portion of the circular fluorescent lamp ofExample 9 of this invention.

In this example, as shown in FIGS. 18 and 19, circular fluorescent lampshaving lamp bases (32, 33) respectively were manufactured. Eithertube-end part 11 or 12 of the circular arc tubes 1 is exposed to theopen air from the lamp base 32 or 33, and a coldest spot was formed onthe exposed tube-end part (11 or 12). The lamp lumen output of thisexample was as same as that of the tubes with the lamp base 29 shown inFIG. 15. Preferably, the coldest spot is formed at the tube-end part 11of the outer circular arc tube 1 as shown in FIG. 18, since thenon-luminous area becomes smaller and a higher lamp lumen output valuecan be obtained.

EXAMPLE 10

FIG. 20 is a partially cutaway view in elevation to show a structurearound the bridge-joint portion of the circular fluorescent lamp ofExample 10 of this invention. As shown in FIG. 20, holding parts (20)for a bending process are provided on the outer surfaces of the bothtube-end parts of the circular arc tubes 1 and 2. And grooves 19 areformed on the inner surfaces of the tips of at least the tube-end parts(11, 12) of the non-electrode side of the tubes 1 and 2. The tube-endparts of the electrodes (3, 4) side of the circular arc tubes (1, 2) aresurrounded with a lamp base 34, while the other tube-end parts (11, 12)are separated from the lamp base 34 and exposed to the open air.According to this example, the holding parts 20 for the bending processcan be held securely during the manufacturing process, and thus theaccuracy for bending process is improved. In addition, coldest spots areformed at the grooves 19, and the temperature can be kept at the bestvalue.

As shown in FIG. 21, the tube-end parts (11, 12) of the non-electrodeside can be sealed with a stem in the same way as the tube-end parts ofthe electrode side. Thus the strength of the tube-end parts (11, 12)will be improved compared to the case shown in FIG. 20. In FIG. 20, thetube-end parts are formed by partially melting the arc tubes. Therefore,tube-end parts (11, 12) of arc tubes with a larger tube diameter becomethinner. On the other hand, if the tube-end parts are sealed with stems,some problems like cracks of the tube-end parts can be prevented duringthe bending process and after the lamp is completed. In addition, themanufacturing method using stems to seal is relatively simple comparedto the case shown in FIG. 20, and thus the tubes can be manufacturedusing conventional facilities.

In this example, the lamp base 34 surrounds only the tube-end parts ofone side of the circular arc tubes 1 and 2. However, this example doesnot limit the structure of the lamp base, so any lamp base with at leasttube-end parts (11, 12) exposed to the open air can be used.

EXAMPLE 11

FIG. 22 is a partially cutaway view in elevation to show a structurearound the bridge-jointed portion of the circular fluorescent lamp ofExample 11 of this invention. As shown in FIG. 22, a lamp base 35 isprovided to surround the tube-end parts of the electrodes (3, 4) sideand the coldest spots of the tube-end parts (11, 12) of thenon-electrode side. And a vent 10 and a thermal shielding plate 16 areprovided for the lamp base 35. The vent 10 allows the coldest spot to beexposed to the open air, and the thermal shielding plate 16 shielded theheat between the tube-end parts of the electrode (3, 4) side and theother tube-end parts (11, 12). According to this structure, it ispossible to prevent the temperature of the coldest spots fromexcessively rising from the best temperature region, because of thesynergistic effect of the vent 10 and the thermal shielding plate 16. Asa result, the lamp lumen output will not be lowered.

EXAMPLE 12

FIG. 23 is a partially cutaway view in elevation to show a structurearound the bridge-jointed portion of the circular fluorescent lamp ofExample 12 of this invention. As shown in FIG. 20, holding parts 20 forbending process are provided to the outer surfaces of the both tube-endparts of the circular arc tubes 1 and 2. And grooves 19 are formed onthe inner surfaces of the tips of at least the tube-end parts (11, 12)of the non-electrode side of the tubes 1 and 2. Other features are thesame as those of Example 11 (FIG. 22), so the explanation is omitted.According to this example, coldest spots are formed on the grooves 19and the temperature is kept at the best value. Thus it is possible toprevent temperature rise of the coldest spots more securely compared tothe case of Example 11. Also, in this example, the holding parts 20 forbending process can be held during the manufacturing process, and thusthe accuracy for the bending process is improved.

As shown in FIG. 24, if the tube-end parts (11, 12) of the non-electrodeside are sealed with stems in the same way as the tube-end parts of theelectrode side, the strength of the tube-end parts (11, 12) will beimproved compared to the case shown in FIG. 23. In FIG. 23, the tube-endparts are formed by partially melting the arc tubes. Therefore, tube-endparts (11, 12) of arc tubes with a larger tube diameter become thinner.On the other hand, if the tube-end parts are sealed with stems, someproblems like cracks of the tube-end parts can be prevented during thebending process and after the lamp is completed. In addition, themanufacturing method using stems to seal is relatively simple comparedto the case shown in FIG. 23, and thus the tubes can be manufacturedusing conventional facilities.

The invention may be embodied in other forms without departing from thespirit or essential characteristics thereof. The embodiments disclosedin this application are to be considered in all respects as illustrativeand not restrictive, the scope of the invention is indicated by theappended claims rather than by the foregoing description, and allchanges which come within the meaning and range of equivalency of theclaims are intended to be embraced therein.

What is claimed is:
 1. A circular fluorescent lamp which has pluralcircular arc tubes disposed concentrically, with a first circular arctube being disposed outwardly of a second circular arc tube, each of thetubes having an electrode at one tube-end part and a sealed portion atthe other tube-end part, and the tubes are joined near the sealedportion with a bridge-jointed portion to form a discharge path insidethe tubes, wherein a distance L₁ (mm) is measured along a center line ofthe first tube from the point corresponding radially to the end part ofthe bridge-jointed portion to the sealed tube-end part of the first tubewhile a distance L₂ (mm) is measured along a center line of the secondtube from the point corresponding radially to the end part of thebridge-jointed portion to the sealed tube-end part of the second tube,and L₁ is longer than L₂.
 2. The circular fluorescent lamp according toclaim 1, wherein the plural circular arc tubes are disposed on the sameplane.
 3. The circular fluorescent lamp according to claim 1, wherein L₁≧1.3L₂ holds true.
 4. The circular fluorescent lamp according to claim1, wherein the tube-end part of the electrode side of the first circulararc tube is longer than that of the second circular arc tube.
 5. Thecircular fluorescent lamp according to claim 1, wherein theconfigurations of the tips of the bridge-jointed portions areessentially symmetric to the center axis of the circular arc tubes. 6.The circular fluorescent lamp according to claim 1, wherein holdingparts for a bending process are provided at the outer surfaces of theboth tube-end parts of the circular arc tubes.
 7. The circularfluorescent lamp according to claim 6, wherein grooves are formed on atleast the inner surfaces of the tips of the non-electrode sides of thetubes.
 8. The circular fluorescent lamp according to claim 1, wherein alamp base is provided to surround the tube-end parts of the electrodeside of the circular arc tubes.
 9. The circular fluorescent lampaccording to claim 1, wherein a lamp base is provided to surround theboth tube-end parts of the circular arc tubes while the tube-end partsof the non-electrode side of the first arc tube are exposed to open air.10. The circular fluorescent lamp according to claim 9, wherein athermal shielding part is provided for the lamp base in order to shieldthe heat between the tube-end parts of the electrode side and thetube-end parts of the non-electrode side.
 11. The circular fluorescentlamp according to claim 1, wherein the tube-end parts of thenon-electrode side of the circular arc tubes are sealed with stems. 12.A circular fluorescent lamp which has plural circular arc tubes disposedconcentrically, with a first circular arc tube being disposed outwardlyof a second circular arc tube, and each of the tubes has an electrode atone tube-end part and a sealed portion at the other tube-end part, andthe tubes are joined near the sealed portions with a bridge-jointedportion to form one discharge path inside the tubes, and coldest spotsare formed at the tube-end parts on the non-electrode side, wherein whena distance L(mm) is measured along a center line of one of the tubesfrom a point corresponding radially to the end part of thebridge-jointed portion to the sealed tube-end part, and an outerdiameter of said one of the tubes is d(mm), 0.5d≦L≦1.3d holds true. 13.The circular fluorescent lamp according to claim 12, wherein the pluralarc tubes are disposed on the same plane.
 14. The circular fluorescentlamp according to claim 12, wherein holding parts for a bending processare provided at the outer surfaces of the both tube-end parts of thecircular arc tubes.
 15. The circular fluorescent lamp according to claim14, wherein grooves are formed at least at the inner surfaces of thetips of the non-electrode sides of the tubes.
 16. The circularfluorescent lamp according to claim 12, wherein the non-electrodetube-end parts of the circular arc tubes are sealed with stems.
 17. Acircular fluorescent lamp which has plural arc tubes disposedconcentrically, with a first circular arc tube being disposed outwardlyof a second circular arc tube, and each of the tubes has an electrode atone tube-end part and a sealed portion at the other tube-end part, andthe tubes are joined near the sealed portion with a bridge-jointedportion to form one discharge path inside the tubes, and coldest spotsare formed at the tube-end parts on the non-electrode side, and thecircular fluorescent lamp is also provided with a lamp base whichsurrounds at least one side of the both tube-end parts, and the coldestspots are exposed to open air.
 18. The circular fluorescent lampaccording to claim 17, wherein the plural circular arc tubes aredisposed on the same plane.
 19. The circular fluorescent lamp accordingto claim 17, wherein when a distance L(mm) is measured along a centerline of one of the tubes from a point corresponding radially to the endpart of the bridge-jointed portion to the sealed tube-end part and anouter diameter of said one of the tubes is d(mm), 0.5d≦L≦1.3d holdstrue.
 20. The circular fluorescent lamp according to claim 17, whereinthe lamp base is separated from the coldest spots of the non-electrodeside.
 21. The circular fluorescent lamp according to claim 17, whereinthe lamp base surrounds the tube-end parts of the electrode side and thecoldest spots of the other tube-end parts, and a vent is provided on thelamp base so that the coldest spots are exposed to open air.
 22. Thecircular fluorescent lamp according to claim 17, wherein a coldest spotis formed in the first circular arc tube.
 23. The circular fluorescentlamp according to claim 17, wherein holding parts for a bending processare provided at the outer surfaces of the both end parts of the circulararc tubes.
 24. The circular fluorescent lamp according to claim 23,wherein grooves are formed on the inner surfaces of the tips of thenon-electrode sides of the tubes.
 25. The circular fluorescent lampaccording to claim 17, wherein the end parts of the non-electrode sideare sealed with stems.
 26. A circular fluorescent lamp which has pluralcircular arc tubes disposed concentrically, with a first circular arctube being disposed outwardly of a second circular arc tube, and each ofthe tubes has an electrode at one tube-end part and a sealed portion atthe other tube-end part, and the tubes are joined near the tube-endparts of the non-electrode side with a bridge-jointed portion to form adischarge path inside the tubes, while coldest spots are formed at thetube-end parts on the non-electrode side and a lamp base is provided tosurround at least one side of the both tube-end parts, and a thermalshielding part is also provided for the lamp base in order to shield theheat between the tube-end parts of the electrode side and those of thenon-electrode side.
 27. The circular fluorescent lamp according to claim26, wherein the plural circular arc tubes are disposed on the sameplane.
 28. The circular fluorescent lamp according to claim 26, whereinwhen a distance L(mm) is measured along a center line of one of thetubes from a point corresponding radially to the end part of thebridge-jointed portion to the sealed tube-end part and an outer diameterof said one of the tubes is d(mm), 0.5d≦L≦1.3d holds true.
 29. Thecircular fluorescent lamp according to claim 26, wherein a lamp base isprovided to surround the tube-end parts of the electrode side and thecoldest spots of the other tube-end parts, and a vent is formed on thelamp base so that the coldest spots are exposed to open air.
 30. Thecircular fluorescent lamp according to claim 26, wherein holding partsfor bending process are provided to the outer surfaces of the both endparts of the circular arc tubes.
 31. The circular fluorescent lampaccording to claim 30, wherein grooves are formed at least at the innersurface of the tips of the non-electrode side.
 32. The circularfluorescent lamp according to claim 26, wherein the tube-end parts ofthe non-electrode side of the circular arc tubes are sealed with stems.